Floods

Floods occur in all 50 states, kill scores of people each year, and cost upwards of six billion dollars annually. On average, most river systems flood every year or two when water leaves its confining channel and flows outward onto the adjacent floodplain. There are two types of floods: regional floods that last for weeks or months, and flash floods that occur suddenly and last only hours. Both floods are dangerous and capable of doing substantial damage to homes, property and infrastructure.

Arizona Floods

Arizona is home to both regional and flash floods.  While regional floods can involve large and small river systems, it is Arizona’s perennial rivers – the Colorado, Salt, Gila and Verde Rivers – that are most heavily impacted.  Arizona’s monsoon season, with its predilection for sudden, torrential and localized rainfall, coupled with a landscape incised by thousands of washes and gullies, is ripe for dangerous flash floods. Of particular concern from a floodplain-management perspective is flooding on alluvial fans, where channels diverge downstream and broad areas outside of channels may be inundated in large floods.

For an excellent review of the impact of regional flooding, see Kyle House’s review article, Arizona Floods of Jan. & Feb. 1993, recalling the severe statewide flooding linked to 1993's El Nino.  Flood damage that year was estimated at about 50 million dollars.  US Highway 95, just east of Yuma, Arizona, closed for nearly six months as the swollen Gila River slowly drained of flood waters.

Figure. Flash flooding in the Santa Cruz River near Tucson; clip from 2.5 minute U.S. Geological Survey video of flash flooding in the Coronado National Forest.

Flash floods rarely cause major economic impact, but they can be murderous; in July 2017, a family of 10 were swept and drowned by a flash flood in a tributary of the East Verde River near Payson, Arizona.  On 12 August 1997, 11 tourists hiking in Antelope Canyon near Page, Arizona, were killed by a flash flood that had its origin in a distant, unseen cloudburst. Flash floods commonly accompany Arizona’s monsoon season, usually driven by local, short-lived rainfall events. 

If you encounter a flash flood: stay safe, stay out of the water.  The Arizona Legislature enacted the “Stupid Motorist” law (statute 28-910) to penalize drivers who drive around barricades in a foolish attempt to cross flooding streams.